why would THREE.ColladaLoader halt all activity on the site?

I am loading an animated .dae file, and I've used the 'onProgress' argument to see it's loading progress. The file loads to 74% in 2 seconds, but then the whole page is paused for 23 more seconds before the model actually loads (and all other activity pauses including css animations). I added a setInterval() to show the progress, but that also gets halted as soon as the loader reaches 74%.
On page load, this is my code:

That is because JS is single theaded and once the file is loaded, what is asynchronous, it starts to parse it and creates everything, what is called a "blocking" process, because it uses the one thread which is available for JS until it is done. Afterwards your animation is resumed. While such a process is running, the UI is frozen, or blocked. That happens because the processing time is much longer than the duration of a single frame (1000/60ms), so the browser cannot update the ui an all other animations.

The only way out of it, is to split the parse and creation task into multiple smaller ones, which can be executed one by one each animation frame, or with a timeout. That way you running animation will gain some processing time in the meantime.